Light-water-cooled nuclear reactors and in particular pressurized-water-cooled nuclear reactors use fuel assemblies comprising a bundle of fuel rods held in place by a framework in mutually parallel arrangements.
The fuel assembly framework comprises, in particular, a plurality of spacer grids distributed along the length of the fuel assembly, in the axial direction of the bundle of rods. The spacer grids ensure transverse retention of the rods and comprise a set of juxtaposed cells arranged in a regular lattice, generally with square lattice cells, the cells themselves having a square cross section in a transverse plane of the spacer grid perpendicular to the axis of the rods. Each of the cells is bounded and separated from the adjacent cells by a peripheral wall having the shape of the lateral surface of a square-based parallelepiped. The cells of the spacer grid are open at their two ends in the axial direction of the cells, so that each of the cells can receive a rod in a central position, in which the axis of the rod is placed along the axis of the cell. The fuel rods have a substantially smaller diameter than the sides of the square cells, so that a free space remains around the rod, between the cylindrical external surface of the rod and the peripheral wall of the cell in which the rod is engaged axially. The spacer grid includes, in each of the cells intended for housing a fuel rod, bearing and retaining means intended to come into contact with the external surface of the fuel rod, in order to ensure that the rod is held in place in transverse directions perpendicular to the axis of the rod and of the cell, while still permitting the rod to move in the axial direction, for example owing to the effect of expansion inside the core of the nuclear reactor.
The means for bearing on and retaining the fuel rods in the cells of a spacer grid generally comprise dimples projecting toward the inside of the cell relative to the peripheral wall, which are produced by cutting and pushing back the wall, and resilient leaf springs, also projecting toward the inside the cell relative to the wall, which are produced by cutting and pushing back a part of the wall, or which are added and fastened to the peripheral wall.
In general, two dimples are provided on each of two adjacent faces of the parallelepipedal peripheral wall and two leaf springs on the faces opposite the faces having the dimples. The fuel rod is contained inside the cell at six points, by the four dimples and the two leaf springs, the leaf springs and the dimples being produced so as to come in contact with the fuel rod in a region of small area, that can be likened to a contact point.
In addition to the transverse retention of the rods of the fuel assembly bundle, the spacer grids ensure mixing and stirring of the reactor cooling water that flows in contact with the fuel rods along the axial longitudinal direction.
The spacer grids for the fuel assemblies consist of metal plates of rectangular shape that are cut at mid-width in order to fit one over the other and assembled by welding, in the form of a lattice of square lattice cells.
To ensure that the cooling water is mixed and stirred upon leaving the spacer grid, the plates of the spacer grids are cut along their edge intended to come into the upper part of the spacer grid, in order to constitute mixing vanes that are folded over toward the inside the cells, alternately in one direction and in the other, so as to guide the fluid leaving one cell into a neighboring cell, and thus mixing the various fluid flows in contact with the fuel rods.
The plates used to produce a spacer grid must therefore be cut in a particular design and stamped or pushed back in order to form the dimples and leaf springs. In the case of leaf springs attached to the walls of the cells, it is also necessary to provide cuts for fastening the leaf springs and then to fit and weld the leaf springs, which may be made of a metal material different from that of the plates.
For example, the plates may be made of a zirconium alloy and the leaf springs of a nickel alloy.
Construction of the spacer grids is therefore complex, because they requires many cutting, stamping, fitting and welding operations.
In addition, the six-point retention of the fuel rods inside each of the cells may result in local wear of the rod cladding in contact with the dimples or leaf springs, by friction because the reactor cooling water sets the rods in vibration inside the spacer grids. This phenomenon, which is generally called “fretting wear”, may result, over long periods of use of the nuclear reactor, in the cladding of the rods being punctured in the contact regions.
Finally, the ability of the vanes cut along the upper edges of the spacer grids to provide mixing is limited, so that perfect temperature homogenization of the cooling water flowing in the fuel assembly is not achieved. This may result in hot spots in contact with the rods in certain regions, where the boiling crisis phenomenon, that is to say the local formation of vapor bubbles in the cooling water, is observed to occur.